It is known that various mechanisms have been used for reciprocating a piston of a pump. Variable stroke-length or amplitude has also been provided in prior drive mechanisms for this purpose, as shown in U.S. Pat. Nos. 3,614,896 and 3,706,233. In these mechanisms, however, the change in stroke-length is rather complicated. To greatly lengthen or shorten the stroke required extended adjustment through a range of intermediate positions as by manual turning of a knurled adjusting knob (for example Item 65 in U.S. Pat. No. 3,614,896) or of a suitable key (for example fitting slot 426 and pin 427 of the embodiment of FIG. 7 in U.S. Pat. No. 3,706,233). Also, in these earlier systems, the piston start position changed with each change of stroke-length. The change from one stroke to another much different in length, whether shorter or longer than the first, practically required discarding the liquid pumped during the transition or providing a special receiving vessel to catch such liquid, unless the drive mechanism is stopped, that is, rendered inoperative, during the adjustment in stroke-length.